indicator (
http://definr.com/indicator)
n 1: a number or ratio (a value on a scale of measurement)
derived from a series of observed facts; can reveal
relative changes as a function of time [syn:
index, {index
number},
indicant]
2: a signal for attracting attention
3: a device for showing the operating condition of some system
4: (chemistry) a substance that changes color to indicate the
presence of some ion or substance; can be used to indicate
the completion of a chemical reaction or (in medicine) to
test for a particular reaction